The invention relates to the field of counter-current flow cooling devices, more particularly to thermoelectric cooling devices.
Temperature is a crucial parameter in an enormous number of physical, chemical and biochemical processes and particularly in a variety of medical and electronic devices that can be operated more effectively at very cold temperatures. While thermoelectric coolers currently in use can readily reach and maintain temperatures in range of 300 K (room temperature) to 230 K, there is no solid-state cooler capable of reaching temperatures below 160 K.
Thermoelectric coolers, also known as Peltier coolers, have existed for many decades, but they have been unable to achieve temperatures cooler than about 210 K primarily because their efficiency drops in inverse proportion to the temperature difference across them. This fact is partly due to the temperature dependence of the properties of thermoelectric materials, but is also largely due to the traditional xe2x80x9cbrute forcexe2x80x9d structure of refrigeration devices including Peltier coolers
FIG. 1 illustrates a standard Peltier cooler designed to reach low temperatures (xcx9c200 K). It consists of a cascade of zigzag structures of junctions between n-type and p-type semiconductors, sandwiched between ceramic plates. When a current flows through the structure, its top face absorbs heat from the environment and its bottom face releases heat to the environment. In other words, the device pumps heat from one face to the other.
Several conflicting processes are at work in this type of Peltier cooler. The current flow pumps heat as a result of the Peltier effect, but heat is generated by the I2R resistive heating. As heat is pumped, a temperature difference builds between the two faces of the device, so the Seebeck effect generates a voltage which opposes the current creating the temperature difference. Ordinary thermal conduction also allows some heat to flow back toward the cold side. The Thompson effects nearly cancel out in this device, so the Thompson effect is usually ignored.
The maximum temperature difference that can be developed by a standard single-stage Peltier cooler with no heat load is about 70 degrees Centigrade. Larger temperature differences, up to 140 degrees Centigrade, can be attained in multistage devices like that illustrated in FIG. 2. However, the pumping efficiency becomes very poor because each stage not only pumps heat that must be pumped in turn by the next stage, but each stage also generates resistive heat that must be pumped in turn by the next stage.
From a different art, in the design of ordinary fluid heat exchangers used in the heating industry it is standard practice to run fluid in opposite directions through two pipes in thermal contact as illustrated in FIG. 3. This works much better than moving the fluid in the same direction through the two pipes. A significant feature of fluid counter-flow heat exchangers is that the temperature difference between the two pipes is nearly zero everywhere along the exchanger, even though there can be a very strong temperature gradient along the length of the pipes.
Fluid counter-flow also occurs in the natural world where a continuous loop may form a fluid counter-flow exchange amplifier, which is essentially a counter-flow exchanger in which the fluid flows as illustrated in FIG. 3, but in which a component of the fluid is separated from the incoming flow and pumped across to the outgoing flow as indicated in FIG. 4. This occurs, for example, in the ocean, where nutrients are concentrated at the shoreline by a counter-flow process. The incoming fluid flows toward the shore along the bottom carrying nutrients, is warmed and flows away from the shore along the surface while gravity pulls the nutrients down to the incoming flow from the outgoing flow, trapping the nutrients in a loop. In another example, one mechanism by which living organisms maintain large ion concentration gradients in certain tissues such as the kidney, is by counter-flow amplification of the solute concentration in fluids flowing across semi-permeable membranes that connect kidney nephrons to blood vessels in counter-directional flow.
There is a need in the art to provide a thermoelectric cooler of a new design which can overcome the limitations of previous coolers and avoid some of the constraints that material properties impose on thermoelectric cooling. Further, there is a need to provide miniature thin film solid state coolers that are useful in computer applications.
The present disclosure fulfills these needs and others that will become apparent from the present description.
In one aspect there is provided a temperature control system, that includes :a counter-current heat exchanger having a first conduit that conveys a heat carrying medium along a forward path from a warm zone toward a cool zone, and a second conduit that conveys a heat carrying medium along a reverse path from the cool zone toward the warm zone, the reverse path being anti-parallel to the forward path. A plurality of heat pumps are distributed along the lengths of the first and the second conduits and are configured to pump heat from a first plurality of points along the forward path to an adjacent second plurality of points along the reverse path. A pump, such as a fan, a compressor or other device is attached to one of the conduits to urge a flow of fluid in at least one of the first and second conduits.
In certain embodiments, the heat carrying medium in at least one of the first and second conduits is a gas, a liquid, a vapor or an electric current. The heat carrying medium in the first conduit may be the same as in the second conduit, or may be a different substance, or the same substance in the same state. In various embodiments, the heat containing medium in one conduit has a different heat capacity than the heat containing medium in the other conduit. When the heat carrying medium is an electric current, the first conduit is a first conductor and the second conduit is made of a second conductor different from the first conductor. For example, one of the first or the second conductors may be p-type semiconductor and the other conductors may be an n-type semiconductor.
In certain embodiments the heat transfer system is configured to exchange a fluid between a first warm volume of fluid and a second cool volume of fluid. One example of this embodiment includes an air conditioning system that exchanges air between an indoor and outdoor volume while pumping heat between the air flowing in the first and the second conduits to maintain a selected temperature in the cool zone with efficient use of energy. More generally, in these embodiments the heat carrying medium in the first conduit is a fluid such as a gas, the first conduit has a first input port to receive an input of fluid from in the vicinity of the warm zone and an output port to convey the fluid to a second volume of fluid in the vicinity of the cool zone. The second conduit has a second input port to receive an input of fluid from the second fluid volume and a first output port to convey the fluid to the first volume of fluid. The plurality of heat pumps distributed between the first and second conduits pump heat from the internally directed warm air to the externally directed cooler air so, that the warm air is efficiently cooled by having its heat incrementally transferred by the counter flow arrangement.
In various embodiments, the heat pump is a Peltier junction, however in other embodiments, the heat pump may be any device that actively pumps heat from the first to the second conduit. Any of the foregoing embodiments may further include a controller that includes a sensor for detecting the temperature in the cool zone and which further outputs control signals to regulate the plurality of heat pumps responsively to the detected temperatures to maintain a desired temperature at one of the two zones. The controller may also be configured to receive a plurality of signals corresponding to the plurality of temperatures at the plurality of points along the first and the second conduits and also configured to regulate the plurality of heat pumps responsively to the plurality of temperatures. In typical embodiments, the controller also regulates a pump, such as fan that urges the flow of the fluid in at least one of the first conduit and the second conduits.
In another aspect, there is provided a Peltier cooling device that includes a first conductor configured to conduct a current flow in a forward direction from a warm zone to a cool zone, and a second conductor different from the first conductor and configured to conduct current in a reverse direction substantially anti-parallel to the forward direction from a cool zone to the warm zone. An electrically conductive junction between the first conductor and the second conductor is positioned at the cool zone (or the warm zone) and a thermally conductive, electrically insulating junction is positioned between the first and second conductors along a portion of their length. In this aspect, the heat is actively pumped across the thermally conductive junction while the current flows in one direction, and then reverses flow at the electrical junction to flow in the anti parallel direction.